while studying pysam issues I found https://github.com/pysam-developers/pysam/issues/592 . Bacically, our ebuilds should do
--enable-gcs --enable-plugins --enable-s3
so that we have less troubles elsewhere. Or at least, provide USE flags which could be depended on by pysam. I wonder whether the cloud-related options require curl, so maybe --enable-libcurl as well?